You can just give a certain amount every year, but it is done with a contract, but you would have to contact the department for details.

I was once on the National Advisory Board and we tried hard to get each BYU grad to just give $10/month back to the university for the church paying 50% of their tuition. They just don't feel the need...but they really do need to do that. There may come a time when the church can't kick in the 50%....I wonder what would happen if tuition just doubled one year. There should be a goal to endow the entire student body tuition over the next 100 years, but Mormons are mostly short sighted and cheap suckers.